.start{
    width: 200px;
    height: 200px;
    background-color: pink;
    text-align: center;
    line-height: 200px;
    position: fixed;
    top: 50%;
    left: 50%;
    margin-top: -100px;
    margin-left: -100px;
}

.start:hover{
    cursor: pointer;
}